490?-420? B.C.

Greek Philosopher

One of the many Greek natural philosophers who benefited from the flowering of culture that accompanied Pericles's reign, Oenopides is perhaps best known for his contributions to mathematical astronomy. Among his accomplishments are determining the angle of Earth's axis relative to the plane of the ecliptic, accurately reporting the length of the lunar month, and calculating the length of the "Great Year," during which the Sun, Moon, and planets all return to the same relative locations in the sky.

Almost nothing is known about Oenopides's early life except that he was born on the island of Chios. He may have visited Athens at some point, but even this is not certain. However, knowledge of his scientific contributions is more certain.

Perhaps Oenopides's best-known contribution to science is his estimate of the angle at which Earth's axis is tilted with respect to the plane...